The invention relates to the field of optical modulators, and in particular to a high speed and low loss GeSi/Si electro-absorption light modulator.
Si-based modulators are highly required for photonic integrated circuits. However, the indirect bandgap of Si prevent any electric-field induced light modulation. Ge-rich GeSi (Ge composition>50%) devices grown on Si are becoming more utilized because of the relatively small difference between the indirect and direct bandgaps of Ge, which can address some of the difficulties Si devices have faced. However, adequate device design and fabrication are very important in forming EA modulators which work efficiently.
There are several challenges in making workable GeSi/Si modulators. The key goal of GeSi based electro-absorption (EA) modulators is to achieve lowest possible loss at voltage off-state with highest possible extinction ratio at voltage on-state. Because GeSi EA modulators inevitably have material loss due to the absorption of indirect band gap, the length of GeSi active region has to be very short, typically less than the order of ˜150 μm, to achieve low insertion loss at off-sate. Therefore, it has to be coupled to a low loss waveguide, such as Si or SiNx wavguides, for on-chip applications. Since GeSi/SiO2 material system has a high index contrast and its single mode dimensional is very small (the single mode cut-off dimension is less than 1 μm), it is a big challenge to achieve an efficient waveguide-modulator coupling. Another issue is that standard reactive ion etching (RIE) of GeSi material usually results in rough sidewalls that increases the scattering loss in the GeSi EA modulator. It would be desirable to circumvent this issue.